The present invention relates to a method in a internal combustion engine for controlling a fuel injector of the unit injector type, which comprises a plunger which is driven by a rotating camshaft and during a working stroke generates a hydraulic pressure increase in the fuel, which is regulated with regard to the pressure build-up by a spill valve and with regard to the injection sequence by a needle control valve.
In an internal combustion engine, such as a diesel engine with unit injectors, the injector plunger requires a strong return spring, in order to retard the plunger over a part of its working stroke. If the force of the return spring is not sufficiently great, there is a risk that the contact in the force transmission chain between the plunger and the driving cam lobe will be broken, with the result that damage can occur, for example, in the immediate area of the injector cam lobe corresponding to the maximum lift position of the plunger when the parts driven by the cam lobe are braked and the inertial forces tend to reduce the contact force between the cam lobe and the cam follower. It is therefore desirable to increase the contact force.
It is important to be able to inject fuel into the engine cylinders over a sufficiently wide crankshaft angle interval, both in order to be able to inject a sufficient quantity of fuel, and also in order to obtain the maximum possible freedom to vary the injection timing (as a function of the engine speed and load, for example). Usually, the crankshaft angle interval over which it is possible or desirable to inject fuel is partially limited by the admissible cam retardation. The admissible cam retardation is in practice determined by the force supplied by the return spring of the injector plunger and the pressure acting in opposition to the plunger; the magnitude of the cam retardation cannot be equal to or greater than this retardation, since the contact between the cam follower and the cam lobe is then broken.
An accepted solution to the problem described above has hitherto been to use a stronger plunger return spring. A disadvantage with a stronger return spring is that such a spring takes up more space. In modern diesel engines the space around the injectors is severely restricted by bulky valve mechanisms. It is therefore advantageous if the dimensions of the return spring can be limited, so that it does not interfere with other functions in the space around the injectors.
It is desirable to provide a method which will permit a rapid retardation of the injector plunger, without the need for a high spring force on the part of the plunger return spring. According to an aspect of the present invention, a method in a internal combustion engine for controlling a unit injector type fuel injector, the fuel injector comprising a plunger which is driven by a rotating camshaft and during a working stroke generates a hydraulic pressure increase in the fuel, comprises regulating hydraulic pressure increase in the fuel with regard to pressure build-up with a spill valve, regulating an injection sequence with a needle control valve, regulating the spill valve and the needle control valve such that additional plunger retardation force is produced in a part of a working stroke in addition to a normal retardation force that is produced by a plunger return spring and flow resistance in the injector, and using a camshaft to extend a crankshaft angle interval for injection with a required plunger speed beyond the crankshaft angle interval used in a normal injector without additional plunger retardation force.